34th INTERNATIONAL CONFERENCE ON
PRODUCTION ENGINEERING
28. - 30. September 2011, Niš, Serbia
University of Niš, Faculty of Mechanical Engineering
MODEL FOR OPERATING COSTS OF PLASMA CUTTING
Srdjan T. Mladenovic, Miroslav R. Radovanovic
University of Nis, Faculty of Mechanical Engineering, Nis, Serbia
[email protected], [email protected]
Abstract: Operating costs of plasma cutting should form the basis for evaluating its profitability.
Acceptable cut quality, increased traverse speed and lower cost per meter of the cut together with
cheaper equipment and possibility of cutting various materials assure wider application of this
procedure. Optimizing a plasma cutting operation based on operation cost is typically a trial-and-error
process that is usually inspired in recommendations given by manufacturers of plasma cutting tools and
consumables. The operating costs for plasma cutting are presented in this paper.
Key words: Advance machining, Plasma Cutting, Operating Cost
1. INTRODUCTION
The plasma-arc process had its origin almost 70 years
ago. In 1941 the U.S. defence industry was looking for
better ways of joining light metal together for the war
effort and, more specifically, for the production of
airplanes. Out of this effort, a new welding process was
born. An electric arc was used to melt the metal, and an
inert gas shield around the arc and the pool of molten
metal was used to displace the air, preventing the molten
metal from picking up oxygen from the air. This new
process "TIG" (Tungsten Inert Gas) seemed to be a
perfect solution for the very specific requirement of highquality welding.
By 1950, TIG had firmly established itself as a new
welding method for high-quality welds on exotic
materials. While doing further development work on the
TIG process, scientists at Union Carbide's welding
laboratory discovered that when they reduced the gas
nozzle opening that directed the inert gas from the TIG
torch electrode (cathode) to the work piece (anode), the
properties of the open TIG arc could be greatly altered.
The reduced nozzle opening constricted the electric arc
and gas and increased its speed and its resistive heat. The
arc temperature and voltage rose dramatically, and the
momentum of the ionised and non-ionised gas removed
the molten puddle due to the higher velocity. Instead of
welding, the metal was cut by the plasma jet.
In Figure 1, both arcs are operating in argon at 200 amps.
The plasma jet is only moderately constricted by the 3/16
inch (4.8 mm) diameter of the nozzle orifice, but it
operates at twice the voltage and produces a much hotter
plasma arc than the corresponding TIG arc. If the same
current is forced through a nozzle with an even smaller
opening, the temperature and voltage rise. At the same
time, the higher kinetic energy of the gas leaving the
nozzle ejects the molten metal, creating a cut.
Fig.1. TIG arc and plasma arc
2. PLASMA CUTTING
Plasma cutting is a process that is used to cut steel and
other metals of different thicknesses (or sometimes other
materials) using a plasma torch. In this process, an inert
gas (in some units, compressed air) is blown at high speed
out of a nozzle; at the same time an electrical arc is
formed through that gas from the nozzle to the surface
being cut, turning some of that gas to plasma. The plasma
is sufficiently hot to melt the metal being cut and moves
sufficiently fast to blow molten metal away from the cut.
The characteristics of the plasma jet can be altered greatly
by changing the gas type, gas flow rate, arc current, arc
voltage and nozzle size. For example, if low gas flow
rates are used, the plasma jet becomes a highly
concentrated heat source ideal for welding. Conversely, if
the gas flow rate is increased sufficiently, the velocity of
the plasma jet is so great that it ejects molten metal
created by the hot plasma arc and cuts through the
workpiece.
Plasma cutting is an industrial process that is essentially
controlled by the operators’ empirical mind-set, which is
typically inspired in recommendations given by the
manufacturers of the cutting torches that are to be used.
Those recommendations, however, reflect the point of
view of the manufacturers’ business, which includes not
only selling the cutting torches but also the consumables.
Yet, the manufacturers’ recommendations usually lead to
solutions that are technically sound in terms of cutting
quality, but do not necessarily correspond to the most
cost-effective solutions on the user’s point of view.
As a result, the user customarily attempts to optimize the
cutting operations by trial-and-error every time it is
needed to setup the existing equipment for a new different
task.
In Table 3 is shown power supply machine.
Table 1. Gas quality and pressure requirements
Plasma
gas
Quality
Pressure
+/- 10%
Flow
rate
O2
Oxygen
99.5% pure
Clean, dry, oil-free
827 kPa /
8.3 bar
4250 l/h
N2
Nitrogen
99.9% pure
Clean, dry, oil-free
827 kPa /
8.3 bar
7080 l/h
Air
Clean, dry, oil-free
827 kPa /
8.3 bar
7080 l/h
(H35 = 65% Argon,
35% Hydrogen)
827 kPa /
8.3 bar
4250 l/h
(F5 = 95% Nitrogen,
5% Hydrogen)
827 kPa /
8.3 bar
4250 l/h
H35
Argonhydrogen
F5
Nitrogenhydrogen
Table 2. Gas types and amperage of current for material
types
Mild steel /
Stainless
steel /
Aluminium
Fig.2. Plasma cutting
In an industrial point of view, general contributions for
the systematization of knowledge on the plasma cutting is
an industrial process that is plasma cutting process appear
to be out of question essentially controlled by the
operators’ empirical since plasma torches and respective
nozzles come in a mind-set, which is typically inspired in
recommenda-wide range of sizes. Additionally, the
topology of operations given by the manufacturers of the
cutting complete plasma cutting systems varies from the
torches that are to be used. Those recommendations,
simple hand-held torches to complex CNC machines
however, reflect the point of view of the manufacture-of
different shapes and sizes.
There are several methods of plasma cutting. The wellknown are: conventional plasma cutting, dual flow plasma
cutting, air plasma cutting, oxygen plasma cutting,
underwater plasma cutting and other.
Gas types
Plasma
Shield
Cutting
30 to 45 A
O2 / N2 & F5 / Air
O2 / N2 / Air
Cutting
80 A
O2 / F5 / -
Air / N2 / -
Cutting
130 A
O2 / N2 & H35 /
H35 & Air
Air / N2 / N2 & Air
Table 3. Power supply
General
Maximum OCV (U0)
311 VDC
Maximum output current (I2)
130 Amps
Output voltage (U2)
50 – 150 VDC
Duty cycle rating (X)
100% @ 19.5 kW, 40°C
Power supplies will
operate between -10°C
and +40°C
0.88 @ 130 ADC output
Ambient temperature/Duty
cycle
Power factor (cosϕ)
Cooling
Insulation
In Table 1 are shown gas quality and pressure
requirements for machine HyPerformance plasma
HPR130.
In Table 2 are shown gas types and amperage of current
for material types of plasma cutting.
Forced air (Class F)
Class H
Input power (input voltage (U1) X input current (I1)
200/208 VAC, 3-PH, 50-60 Hz, 62/58 Amps
400 VAC CE, 3-PH, 50-60 Hz, 32 Amps
3. COST OF PLASMA CUTTING
C P  cP  QP
(4)
How to calculate cost of operation and establish metrics
for improvement? There are many costs associated with a
mechanized plasma-cutting machine beyond the capital
equipment purchase. There are general overhead costs,
maintenance costs, service call charges, gas costs,
consumable and torch costs, and electricity charges. The
plasma-profiling machine is also likely to have a host of
auxiliary equipment that may also be considered: material
handling equipment, environmental control equipment,
safety gear etc. The labor component for plasma cutting
may include machine operators, helpers, maintenance
personnel, secondary operation workers and others. The
intent of this article is to review the most significant
variables affecting annual cost of operation and to
establish metrics for improvement.
CS  cS  QS
(5)
In typical plasma cutting operations there are four major
ongoing costs: cost of power, gas, cost of consumables,
and cost of labor.
Productivity can be regarded as being the ratio between
production speed and cost. For plasma cutting,
productivity can be defined by an expression of the type
P
VC
CH
(1)
Where VC is the traverse speed and CH is the cutting cost
per unit time.
Both VC and CH depend on several process variables and
productivity can be improved either by increasing the
traverse speed, or decreasing the cutting cost per unit
time, or both.
For a given torch, the main process variables in plasma
cutting are the amperage the current, the traverse speed
VC, the pressure of the cutting gas, and the pressure of the
protective gas. There are four major factors for the
production cost in typical plasma cutting operations:
electrical power, gases and torch consumables [1].
C H  C E  C P  CS  CM
(2)
Where CE, CP, CS and CM are respectively: the cost per
unit time of the electrical power, the cost of the cutting
gas (plasma), the cost of the compressed air that is used as
protective gas (shield) and cost of the torch consumables.
All the costs are expressed in €/h.
Cost of the electrical power can be defined by expression:
CE  cE  PE  cE  PP  PT 
(3)
Where cE is unit cost of electric energy (EUR/kWh), PE is
electric power consumption (kW), PP is electric power of
aggregate the plasma, PT is electric power of working
table.
Cost of the cutting gas (plasma) and the cost of the
compressed air that is used as protective gas (shield) can
be defined by expressions:
Where cP and cS is unit cost of plasma gas and shield gas
(EUR/m3), QP is gas consumption (m3/h).
Cost of the torch consumables CM to set a monthly or year
monitoring of consumption, and later this value is
translated into cost per time.
4. OERATING COSTS
The major power consumer in a cutting machine is the
DC power supply. Most of the energy consumed by the
system is put directly to work on the material in a very hot
energy-dense arc. To get a rough idea of the power
consumption of plasma system is multiply the amperage
output by the average operating voltage. To calculate
kilowatts of input consumed, multiply by a power supply
efficiency factor of around 85%. Example an 80A plasma
system has an average operating voltage of about 100V.
This means the power supply puts out 6.8 kW (8kVA x
0.85 = 6.8 kW).
To arrive at daily or yearly power consumption multiply
times the average up-time or arc-on time in a day. Arc-on
time is the amount of time actually spent cutting over a
given time interval. This can be measured by a pierce and
arc-on time counter, or calculated from programming
distances and speeds and daily throughput. Arc-on time
will vary with material type and thickness, size of cut
pieces, material handling, machine speed, torch height
control speed, and many other factors. Most shops
average about 55% actual arc-on time. That means in a
given 8-hour shift only 4.4 hours are spent cutting. In the
year we have 1144 hours are spent cutting (260 days).
Plasma systems use as plasma gas: oxygen, air, nitrogen,
argon-hydrogen, and other gases. The consumption rate
varies with the size of the plasma system and various
operating conditions. Generally the operations manual
will provide consumption rates in cubic meter per hour
for a given nozzle size and operating pressure or flow
tube setting. For example an 80A oxygen plasma system
consumes 2 m3/hours of oxygen when cutting. To find the
cost of operation multiply the consumption rates of
plasma gas by the arc-on time and cost of the gas, which
is often measured in EUR per m3. The same system may
use 8.5 m3/hours of shield air. Shop air is generally
considered free other than associated maintenance costs to
keep it clean. But shield gases such as nitrogen O2, and
mixes can be costly and should be calculated as above.
Consumable costs can be tracked on a weekly, monthly or
yearly basis. These costs vary widely depending not only
on the cost of the parts but on the performance and life of
the parts, which is dependent on many factors.
Consumable and plasma torch life varies with application,
operating parameters, duration of cuts, number of pierces,
operator skill etc. The best way to capture and begin to
control consumable costs is to keep daily logs of parts life
measured in number of pierces and arc hours. Over time,
in a production environment, it is possible to closely track
the number of pierces and the total arc-hours for a given
set of parts on a given cutting job. If a plasma torch is
operated and maintained correctly the annual cost of
torches, gas swirling devices, shields, retaining caps and
other parts should be low compared to the nozzle and
electrode cost. But the reality in many shops is that
overall consumable cost is 2 X the nozzle and electrode
cost.
In Table 4 are shown consumption of the power, cutting
and shield gas in m3 per hour and consumable in EUR per
day.
Table 4. Consumption of power, cutting and shield gas
and consumable
Mild Stainless AlumiFactors
Steel Steel
nium
Power (current 80A) kWh 8.2
7.5
10.2
O2
2
N2
0.8
Cutting gas
(plasma gas)
F5
0.4
H35
0.3
1.5
O2
0.5
Protective gas
N2
4.5
2
(shield gas)
Air
8
6
Consumable
EUR
10
10
10
In Table 5 are shown operating costs for one month if
cutting machine works 25 days (110 hours). A price of
0.105 EUR/kWh was used for the electricity costs. The
following gas prices were taken as the basis for
calculating the plasma gas costs: N2 – 1.5 EUR/m3, O2 1.5 EUR/m3, F5 - 2 EUR/m3 and H35 - 2 EUR/m3. Air is
generally considered free, but consumption of air
compressor during compression to 6-8 bar is about 2
kWh.
Table 5. Operating costs for one month
Mild Stainlees
Factors
Steel
Steel
Power
95
86.5
O2
330
N2
132
Cutting gas
(plasma)
F5
88
H35
66
O2
82.5
Shield gas
N2
990
Air
23
Consumable
250
250
Overall
EUR 780.5
1612.5
Aluminium
118
330
330
20
250
1048
The table shows that the operating costs of a plasma
cutting are big, and it is therefore necessary to optimize
the machine and several recommendations are given in
the conclusion.
5. CONCLUSION
Plasma cutting is a process that is used to cut steel and
other metals. Here are some recommendations for
optimizing plasma cutting machine to lower cost of
operation and increase productivity:
1) Maximize up-time on the machine. A cutting
machine should be cutting. Preventative maintenance is
essential to prevent costly downtime for repairs. Material
handling solutions such as multiple cutting beds,
overhead cranes, and plate handlers can minimize manual
loading and offloading and keep the operator focused on
the cutting process. Motion matters as well: If the torch
height controls or machine traverse speed is slow the
machine spends more time positioning the torch than
cutting metal.
2) Minimize secondary operations: Controlling costs of
secondary options is achieved by optimizing cut quality.
To do this requires not only a well-maintained machine
but also a well-trained operator. The highly skilled
operator produces more cut pieces, of higher quality, with
less scrap material and less rework down the line. Getting
good cut quality from the plasma arc cutting process
requires careful control over process parameters and
attention to detail.
3) Control consumable costs: Controlling consumable
costs, like controlling cut quality is part equipment and
part operator. A good operator will get the most out of a
set of parts and prevent catastrophic failures.
ACKNOWLEDGEMENT
This paper is part of project TR35034 The research of
modern non-conventional technologies application in
manufacturing companies with the aim of increase
efficiency of use, product quality, reduce of costs and
save energy and materials, funded by the Ministry of
Education and Science of Republic of Serbia.
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